\documentclass[12pt]{article}   \textheight 15cm
\begin{document}
\centerline{This line will be centered}

\begin{center}
  This is a section,\\ in which each line\\ will be centered. \par
  Start new paragraphs as usual with empty lines or with \verb+\par+,\\
  make a line break (new line) with \verb+\\+, \verb+\newline+ etc.
\end{center}

\begin{flushleft}
  The following text will be ``ragged-right'' until the end of the 
  current environment.\par
  Start new paragraphs as usual with empty lines or with \verb+\par+,\\
  make a line break (new line) with \verb+\\+, \verb+\newline+ etc.
\end{flushleft}

{\raggedright
  The following text will be ``ragged-right'' until the end of the 
  current environment.\par
  Start new paragraphs as usual with empty lines or with \verb+\par+,\\
  make a line break (new line) with \verb+\\+, \verb+\newline+ etc.
}

\begin{flushright}
  The following text will be ``ragged-left'' until the end of the 
  current environment.\par
  Start new paragraphs as usual with empty lines or with \verb+\par+,\\
  make a line break (new line) with \verb+\\+, \verb+\newline+ etc.
\end{flushright}

{\raggedleft
  The following text will be ``ragged-left'' until the end of the 
  current environment.\par
  Start new paragraphs as usual with empty lines or with \verb+\par+,\\
  make a line break (new line) with \verb+\\+, \verb+\newline+ etc.
}
\end{document}